1. Field of the Invention
The present invention relates to a primary radiator used in a satellite antenna, etc., and, more particularly, to a primary radiator using a dielectric feeder.
2. Description of the Related Art
FIG. 16 is a sectional view of a conventional primary radiator using a dielectric feeder. The primary radiator comprises a waveguide 10 that has an open end and a closed end. The closed end is bounded by a surface 10a. A dielectric feeder 11 is held in an opening 10b of the waveguide 10. Inside the waveguide 10, a first probe 12 and a second probe 13 are positioned orthogonal to each other, and the distance between these probes 12 and 13 and the surface 10a is approximately xc2xc of the guide wavelength.
The dielectric feeder 11 is made of a dielectric material, such as polyethylene. A radiation section 11b and an impedance conversion section 11c are formed at ends of the dielectric feeder 11 which has a holding section 11a as a boundary formed therebetween. The outer diameter of the holding section 11a is nearly the same as the inner diameter of the waveguide 10, and the dielectric feeder 11 is fixed to the waveguide 10 by the holding section 11a. Both the radiation section 11b and the impedance conversion section 11c have a conical shape. The radiation section 11b protrudes outward from the opening 10b of the waveguide 10, and the impedance conversion section 11c extends to an interior of the waveguide 10.
The primary radiator described above is disposed at a focal position of a reflecting mirror of a satellite reflection-type antenna. In this device, radio waves transmitted from a satellite are focused to the inside of the dielectric feeder 11 from the radiation section 11b. Impedance matching is performed by the impedance conversion section 11c of the dielectric feeder 11. The radio waves travel into the interior of the waveguide 10. When the radio waves are received by the first probe 12 and the second probe 13, the received signal is frequency-converted into an IF frequency signal by a converter circuit (not shown).
As illustrated by the dashed line in FIG. 15, the radiation pattern received by the primary radiator described above contains side lobes. The side lobes are formed because a surface current flows to the outer surface of the waveguide 10 and is radiated due to the discontinuity of the impedance that lies within the opening 10b. For example, when the designed radiation angle of the radiation section 11b is 90 degrees (i.e., xc2x145 degrees with respect to the center), high amplitude side lobes are generated in the range of xc2x150 degrees. Because the gain of the main lobe in the central portion of the radiation angle is decreased, the radio waves from the satellite are not received efficiently.
According to a first aspect, a primary radiator comprises a waveguide having an opening at one end that receives a dielectric feeder. The dielectric feeder is held within the waveguide. A radiation section is formed such that a portion protrudes from the opening of the waveguide. An annular wall having a bottom wall and an opening, is provided adjacent to the waveguide. The depth of the annular wall is about xc2xc of the wavelength of the radio waves. Preferably, the width of a bottom surface of the annular wall is about ⅙ to {fraction (1/10)} of the wavelength of the radio waves.
According to a second aspect, the phases of a surface current flowing on the outer surface of the opening of the waveguide and a surface current flowing on the inner surface of the annular wall are about one hundred and eighty degrees out of phase. Accordingly, the currents substantially cancell, the amplitude of the side lobes are greatly reduced, and the gain of the main lobe is increased. Furthermore, if a plurality of annular walls are provided concentrically, the amplitude of the side lobes are also reduced.
According to a third aspect, a primary radiator comprises a waveguide having an opening at one end that receives a dielectric feeder that is held within the waveguide. A radiation section is formed such that a portion protrudes from the opening of the waveguide. A gap having a depth of about xc2xc of the wavelength of the radio waves is provided between an inner wall surface of the opening of the waveguide and the outer surface of the dielectric feeder.
In this aspect, the phases of a surface current flowing on the outer surface of the dielectric feeder and a surface current flowing on the inner surface of the waveguide are substantially out of phase and cancel or substantially cancel each other. As a result, the side lobes are greatly reduced, and the gain of the main lobe is increased.
In a fourth aspect, the gap can be formed by making the opening of the waveguide protrude outward. The gap is formed within recessed sections in which the outer surface of the dielectric feeder is cut out. In this aspect, preferably, the width (i.e., the facing distance between the dielectric feeder and the waveguide) of the gap is about ⅙ to {fraction (1/10)} of the diameter of the opening of the waveguide.
Although the gap can be provided around the entire periphery of the inner wall surface of the opening of the waveguide in the above described aspects, the gap also may be provided in a portion of the inner wall surface of the opening of the waveguide when a symmetry is substantially maintained. In this aspect, preferably, a plurality of recessed sections are formed on the outer surface of the dielectric feeder, and the projection portions between recessed sections are coupled to the inner wall surface of the opening of the waveguide. In this arrangement, the holding strength of the dielectric feeder increases.